"...Web of Science (WOS), published by Thomson Reuters (TR), is a multidisciplinary abstracting and indexing (A&I) service with different components to which libraries and academic institutions can subscribe in combination. Journal indexing is cover-to-cover for 16,959 titles including 726 open access publications... — Goodwin, 2014

Elsevier's Scopus and Thomson Reuter's Web of Science (WoS) are the two most extensive, popular (and commonly-used) search tools in academia to track impact factors. Besides searching the literature, these two databases also rank journals re: productivity and total # of citations to indicate impact, prestige and influence. The principles behind bibliometrics, and cited reference searching, are used by WoS and Scopus to track impact of authors, their scholarship, and where they opt to publish.

Scopus and WoS, though complementary, are very different tools in terms of their coverage/scope and methods. If researchers or librarians were asked to select one, which do they prefer? To answer this, the two databases should be compared head to head for qualitative and quantitative aspects. Both tools make use of bibliometrics but each has unique features, coverage and practices to arrive at citation counts and impact. Scopus ultimately has more content (~18-22,000 journals) but has an obvious bias in its coverage of European journals Elsevier titles. WoS covers about ~12,000 journals (open access titles and conference abstracts) but reveals its own American bias. Academic libraries provide access to either Scopus or WoS, and sometimes both but this is rare.

In further detail, WoS is a multidisciplinary database that contains the Science Citation Index, Social Sciences Citation Index and Arts & Humanities Citation Index. Scopus provides access to scientific, technical, medical and social science literature. While several databases such as EBSCO offer a kind of internal cited reference searching their coverage is not as comprehensive as the WoS. In fact, it can be said with some certainty that no single tool is able to track all citations and the research citing them. The databases that offer cited reference searching often focus on academic journals that they index and neglect papers in the deep web (seegrey literature). As a result, some important seminal articles and monographs are always missed.

Dr. Peter Jacso criticizes the claims of those who use Scopus, WoS and Google scholar pointing out that " ...knowing the bibliometric features of databases, their own h-index and related metrics versus those of the alternative tools can be very useful for computing a variety of research performance indicators. However, we need to learn much more about our tools in our rush to metricise everything before we can rest assured that our gauges gauge correctly or at least with transparent limitations...". In light of the ubiquity of new author impact metrics, his statements have a resounding ring of truth to them. In other words: librarians beware!

Scopus

Subject coverage in Scopus

Scopus indexes 53 million records, 21,915 titles and content from 5,000 publishers, and claims to be the largest abstract and citation database of research literature and quality web sources. This claim is challenged by researchers in various fields including in library science (Jacso, 2011). See this breakdown of Scopus content. Elsevier is the owner of Scopus and is also one of the main international publishers of scientific journals.

Scopus has added over 160 million cited references to its database, dating back to 1970.

Nearly 22,000 titles from 5,000 publishers worldwide

4.9 million conference proceedings, and more than 4150 Open Access journals

100% Medline coverage

20+ million records back to 1996 with references

20+ million pre-1996 records go back as far as 1823

40,000 monographs or books

386 million scientific web pages

22 million patent records from 5 patent offices

Links to full-text & other library resources

Innovative tools to review results and refine to relevant hits

Alerts to keep you up-to-date on new articles matching your search query, or by favorite authors

With Scopus you can construct long-term, extensive bibliometric and historic trend analyses, and more complete author profiles and h-index measurements for individuals who began publishing prior to 1996.

Sciverse Scopus covers 250 million quality web sources, including 22 million patents. Searches in Scopus incorporate searches of scientific web pages through Scirus, and include author homepages, university sites and resources such as preprint servers and OAI compliant resources.

In 2014, Scopus announced a new journal metric called Impact per Publication (IPP). IPP measures the ratio of citations per article published, and provides an additional metric for comparing and evaluating journals. Access the IPP metric from the "Compare journals" tool in Scopus.

Web of Science

Subject coverage in WoS

Thomson Reuter's Web of Science (WoS) (formerly Web of Knowledge) provides access to a network of scholarly articles linked by their references. Articles have been indexed from journals since 1960 and 12,000 journals are currently covered. WoS is the online version of the Science Citation Index with some differences. Separate annual editions covering science, social sciences, and the arts and humanities have been integrated into a multiyear multidisciplinary system. WoS covers nearly 23 million source papers from the 1940s to the present, and frequently updated.

Summary:
Web of Science is updated with approximately 25,000 articles and 700,000 cited references added each week.

Science – 7100 international journals and highly cited book series in 170 categories back to 1900

Social Sciences – 1,750 international journals and highly cited book series in 50 subject categories back to 1954

Arts & Humanities – 1,200 international journals and highly cited book series in 25 categories back to 1975

Complete backfiles to 1945 however put total at ~37 million records

Cited reference and chemical structure searches

Author identification tools

Analysis capabilities

Direct links to your full-text collections

Index Chemicus®: Over 2.6 million compounds, to 1993

Current Chemical Reactions®: Over one million reactions, to 1986, plus INPI archives from 1840 to 1985.

WoS provides unique search methods and cited searching. Users can navigate forward and backward through the literature, and search all disciplines and time periods. Users can navigate to print and electronic collections using institutional linkresolvers.

Web of Science (WoS) is searchable with complete bibliographic data, cited reference data and navigation and links to full text.

See also Essential Science Indicators (and ESI specifics to determine the influential individuals, institutions, papers, publications, and countries in their field of study — as well as emerging research areas that could impact their work

Thomson Reuters Impact Factor

JCR provides quantitative tools for ranking, evaluating, categorizing, and comparing journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. The annual JCR impact factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years.

Substantial differences exist between WoS, Scopus and Google scholar - the latter delivers instant results for searchers. This can (subconsciously) be a major reason for users to choose it over other tools.

Google scholar is much larger than either WoS or Scopus but it has been shown to have fewer references to selected articles. However, GS' unique coverage and web crawling techniques means that it has been shown to have five (5) times as many unique cited items although many counts are inflated.

Google Scholar

Google scholar is easy-to-search, provides fast access into grey literature and other related (and unrelated) cited papers. Jacso says that the poor quality control and inflated citation counts in Google Scholar makes it nearly unusable for bibliometric purposes. It might be better to say that GS should be used but not in isolation with the foreknowledge that it has a number of major limitations. A number of Impact factors - such as the h-index - are now determined by using Google scholar data despite its many limitations, metadata problems and inflated citation counts. Although Google scholar provides access to other papers through its cited by feature it is generally seen to be a browsing or discovery tool not a properly curated bibliometric tool like WoS or Scopus. Reliable bibliometric searching requires better tools that employ cited reference searching based on accurate counts.